1. Field of the Invention
The present invention relates to a pupil measuring system for measuring a pupil diameter and a system for diagnosing Alzheimer's dementia (Alzheimer's disease) by measurement of the size of a pupil.
2. Related Art
It is said that about one million patients suffer from senile dementia in Japan. About half those patients are affected by cerebrovascular dementia, and the rest of them are affected by Alzheimer's disease. No early diagnosing method was conventionally available for Alzheimer's patients. Medicines for delaying the progress of Alzheimer's disease have now been developed, which makes it possible to delay the progress of the disease if Alzheimer's disease can be diagnosed early. For this reason, the method for early diagnosis of Alzheimer's disease has long been expected.
A method for early diagnosis of Alzheimer's disease was reported in a US Journal entitled SCIENCE (VOL. 266, 11 NOV., 1994), which enables the early diagnosis of Alzheimer's disease by dropping a pupil dilating dilution into an eye, and by measuring the area of a resultantly dilated pupil or the magnification of the pupil diameter. However, a diagnosing system employing this method is not available yet.
The following problems were encountered as a result of an attempt to build the diagnosing system of this type by combining together conventional units, such as, a pupil photographing unit which measures the area of a pupil or the magnification of a pupil diameter and a data analyzing unit which analyzes data obtained by the pupil photographing unit.
In the conventional pupil photographing unit, an eye is exposed to continuous light using a continuously illuminating device as a photographing device. The pupil is stimulated by flashing light, and the area of the pupil or changes in the pupil diameter obtained as a result of the lapse of several seconds after the stimulation of the pupil. The above described method for early diagnosis of Alzheimer's disease requires the measurement of changes in the pupil size over a long period of time, e.g., 30 minutes to one hour. However, the eye often moves during the course of the measurement which is continued over such a long period time, thereby resulting in eye deflections. The eye deflections make it difficult to accurately measure the area of the pupil or the pupil diameter.
To reproduce an image on a monitor by processing a signal output from a video camera, the speed of an electric shutter is usually set to a standard speed, that is, a rate of 1/60 sec. Data for two fields are obtained from the data which the video camera outputs for every one screen, that is, at an interval of 1/60 sec. One screen (one frame) is formed on a monitor screen by means of the data. If the eye moves quickly, displacements arise even in the image of the pupil for two fields at an interval of 1/60 sec. This problem will be described in a more detailed manner.
Eye movement is generally divided into two types, i.e., a slow phase and a quick phase. The slow phase designates the slow movement of the eye, and the quick phase designates the quick movement of the eye. The quick phase represents, for example, the momentary return of the eyes after they have followed a train passing a railway crossing from left to right. Contrary to this, the slow phase means the movement of the eyes as they are following the train from left to right. It is said that a rotating speed of the eye in the quick phase is 300 to 500 degrees/sec. Provided that the rotating speed of the eye is 500 degrees/sec., the front surface of the eye moves together with a pupil in the following manner. In the example, the diameter of the eye is assumed to be 30 mm, as shown in FIG. 9, and the speed of the electronic shutter is set to a standard period of 1/60 sec.
(1) A traveling speed of the pupil center is .pi..times.30.times.(500/360) mm/sec.=about 131 mm/sec. PA1 (2) A travel distance of the pupil center during a period of 1/60 sec. is (131 mm/sec).times.(1/60 sec.)=about 2.2 mm. PA1 (3) Displacements of the subject, i.e., the pupil, are as follows:
The pupil commonly measures approximately 3-5 mm in diameter. In this event, the pupil is assumed to measure 4 mm, as shown in FIG. 9. The pupil having a diameter of 4 mm travels 2.2 mm during a period of 1/60 sec, which in turn results in a substantially elliptical image having a longer radius of about 6.2 mm and a shorter radius of about 4 mm, as shown in FIG. 10. An apparent area of the image is about more than 150% of the true area. Hence, there arises an error of more than 50%. For this reason, it becomes impossible to accurately measure the area of the pupil or the pupil diameter.
To prevent such a problem, it may be possible to form one screen (one frame) on the monitor screen by using the data of the identical field obtained from the video camera twice while the release period of the electronic shutter remains set to a standard period of 1/60 sec. Even in the case of this method, the image moves during one field having a period of 1/60 sec. if the eye is exposed to continuous light, thereby resulting in a blurred pupil image.
Alternatively, it may be possible to use intensive and continuous light while the speed of the electronic shutter is set to a small period of time, e.g., 1/1000 sec. According to this method, on the assumption that the rotating speed of the eye is 500 degrees/sec. and the diameter of the eye is 30 mm, the pupil center travels at a speed of about 131 mm/sec along the surface of the eye. Hence, the travel distance of the pupil center during a period of 1/1000 sec. is (131 mm/sec.).times.(1/1000 sec.)=0.131 mm. Assuming that the pupil diameter is 4 mm, a ratio of the apparent area of the pupil to the true area of the same is 4.131.sup.2 /4.sup.2 =1.067 (times). The error of the area of the pupil is reduced to about 6.7%.
However, if the eye is continuously exposed to intensive light, there is a risk of damaging the cornea of the eye. For this reason, this second method is not practical. Conversely, if the speed of the electronic shutter is set to a very short time, e.g., 1/1000 sec., and weak light is continuously used, the image will become dark, which makes it difficult to analyze the image. The dark image is not suitable for measuring the pupil size.
The conventional data analyzing unit for analyzing data obtained by the pupil photographing unit is designed to directly use data relating to the size of the pupil which are obtained at predetermined intervals of time. However, the eye itself is reduced or increased in size all the time. Thus, constant changes of this type in the area of the pupil are ignored in the conventional data analyzing unit, which in turn results in errors. For example, on the assumption that the area of the pupil obtained as a result of the first measurement is 100%, and that the area of the pupil is then measured for 40 seconds at every 0.2 seconds, the area of the pupil changes all the time, as shown in FIG. 7. The area changes in as much as a range of about 100-130%. If the minor movement of the eye is measured for a short period of time, no problems arise. However, this method is not desirable to measure the overall changes in the pupil size over a long period of time, and data obtained as a result of such a measurement make it difficult to carry out a correct diagnosis of Alzheimer's disease.